1. Field of the Invention
The present invention relates to a solder paste composition suitable, when mounting electronic components on an electronic circuit substrate, for precoating the substrate with solder by using a dam. The present invention also relates to a solder precoating method using the solder paste composition.
2. Description of Related Art
Recently, as the miniaturization and the lightening of electronic equipments are enhanced, electronic components to be mounted requires a large number of pins and a fine pitch, and a conductor pattern also increases the tendency of a fine pitch where a large number of conductors are formed at extremely small spaced intervals in a narrow range. Therefore, when the electronic components are connected to an electronic circuit substrate, there is widely employed amounting method using solder bumps instead of a conventional wire bonding.
As a method of forming the solder bumps, a solder precoating method using a resin mask (a dam) is employed (Japanese Unexamined Patent Publication No. 2002-334895). FIGS. 1(a) to 1(c) are process drawings showing the solder bumps forming method by means of the solder precoating method using a general dam, as described in Japanese Unexamined Patent Publication No. 2002-334895.
In this bump forming method, on a substrate 1 coated with a solder resist film 3 provided with opening parts for preventing electrodes 2 formed on the surface of the substrate 1 from being covered, a dam 4 is firstly formed so as to surround the electrodes 2, as shown in FIG. 1(a).
Subsequently, as shown in FIG. 1(b), a solder paste composition 5 containing predetermined solder powder is filled over the electrodes 2 within the opening parts surrounded by the dam 4. This is then heated to have the solder adhere to the surfaces of the electrodes 2, thereby forming solder bumps 6, as shown in FIG. 1(c).
In accordance with the solder precoating method using the above-mentioned dam 4, the solder bumps 6 can be formed with a fine pitch.
However, during the time of heat-melting, the solder powder within the solder paste composition 5 filled within the opening parts surrounded by the dam 4 cannot be deposited satisfactorily on the electrodes 2. This results in the problem that no solder bumps 6 can be formed on the electrodes 2, being called “bump defect.” Only a bump defect on a substrate would ruin the substrate. Hence, there has been a strong desire for the development of a solder paste composition capable of forming the bumps 6 with a high yield.
There has also been a tendency of non-uniformity in the height of the solder bumps 6 formed. It is preferable to form the solder bumps 6 so as to have a uniform height because the uniformity in the height of the solder bumps 6 exerts a great influence on the reliability of junction when mounting the components in the succeeding step.
Compared with the conventional manner using no dam, with the solder precoating method using the dam, the opening parts above the electrodes become thick by the amount of the dam thickness (In general, the thickness of the dam 4 is several times to several tens times the thickness of the solder resist film 3). Consequently, with the method using the dam, the solder powder cannot be deposited satisfactorily on the electrodes 2. This creates the above-mentioned problems not encountered in the conventional manner.
Japanese Unexamined Patent Publication No. 2002-141367 describes a way of forming bumps in a solder precoating method using a mask (a dam), wherein a solder paste contains solder powder, and 10 weight % or below of the solder powder have a particle size of not less than the thickness of the mask (the dam) nor more than 1.5 times the thickness thereof.
This publication No. 2002-141367 describes that, even if a squeegee is repeatedly shifted on the mask in order to ensure the filling of the solder paste into opening parts, the solder powder already filled in the opening parts is less likely to be removed, and the bumps are unsusceptible to variations in dimension. However, this method is not necessarily sufficient to suppress the variations in the bumps. This publication is also silent about the bump defect due to unsatisfactory deposition of the solder powder on the electrodes during the time of heat-melting.